1. Field of the Invention
The present invention relates to an oxygen concentration sensor employed for air/fuel ratio control or the like of an automobile engine.
2. Description of Related Art
An oxygen concentration sensor as described below has been conventionally installed in an exhaust pipe or the like composing part of the exhaust system of an automobile engine to control an air/fuel ratio of the automobile engine.
An oxygen concentration sensor generally includes a sensor element inserted into a housing, a protective cover disposed at an end portion of the housing, a lead wire inserted into the protective cover, and an elastic insulating member disposed inside the protective cover and also having a lead wire insertion hole for receiving the lead wire. The lead wire inserted into the lead wire insertion hole is sealed and fixed into the above-described lead wire insertion hole by caulking it to the protective cover in the radial direction (see, e.g., Japanese Utility Model Laid-Open No. Hei 2-60864).
Recently, however, the fixing position of the above-described oxygen concentration sensor has been varied. For instance, it has been frequently installed at the downstream side of the exhaust pipe. In this case, the oxygen concentration sensor is splashed with water thrown out by a tire or the like during the driving of an vehicle or it may be doused in puddles. If water enters the inside of the oxygen concentration sensor due to dousing or splashing of water, it causes disorders and malfunctions such as deterioration of an output of the oxygen concentration sensor and cracks of the sensor element due to the dousing or splashing of water.
In recent years, moreover, the number of oxygen concentration sensors having a built-in heater in addition to taking out a grounding lead, and laminated oxygen concentration sensors having a two-cell type sensing element including many lead wires has been increasing. Since a large number of the lead wires are disposed inside the protective cover in these oxygen concentration sensors, a larger quantity of lead wire insertion holes have to be disposed in the elastic insulating member.
In the elastic insulating member having the above-described multiple lead wire insertion holes, numerous thin and a thick portions are disposed in close proximity to one another. In such a situation, when a lead wire is inserted into the above-described elastic insulating member and the protective cover is caulked to seal and fix the lead wire on the elastic insulating member, compression stress imposed on the elastic insulating member by caulking the protective cover is concentrated on a thin portion of the elastic insulating member such as an easily deformed portion between the lead wire insertion hole and the outer periphery of the elastic insulating member, so that the compression stress in that portion is increased. On the other hand, the stress in a thick portion such as a central portion of the above-specified elastic insulating member is decreased.
In short, compression stress imposed on each portion in the above-described elastic insulating member may not be even. Accordingly, the oxygen concentration sensor including the elastic insulating member having many lead wire insertion holes has insufficient sealing performance on a contact surface between the lead wires and the lead wire insertion holes. In this case, when the oxygen concentration sensor is splashed with water, the water may easily go into the interior of the oxygen concentration sensor through the above-described lead wire insertion holes.
Furthermore, compression stress imposed on each portion is in an uneven state on the elastic insulating member having many lead wire insertion holes as described above. When such an elastic insulating member is exposed to high ambient temperatures, a large amount of compression stress is imposed on it; thus, a portion thereof is deformed to a great extent and also loses its elasticity and wears out, so that portions in such a state can no longer seal between the lead wire insertion holes and the lead wires.
When an oxygen concentration sensor as described above is employed for controlling an air/fuel ratio in an engine, the exterior temperature of the oxygen concentration sensor comes from radiant heat from the exhaust pipe or the like having high temperature exhaust gas passing therethrough. In the above environment, a problem in which the elastic insulating member looses elasticity in such a high temperature environment is apt to happen more than ever before.